1. Field of the Invention
The present invention relates to a knock control apparatus for an internal combustion engine which detects knocking (hereinafter abbreviated as a “knock”) of the internal combustion engine based on an amount of ions (ionic current) generated upon combustion of the internal combustion engine, and corrects control parameters (ignition timing, etc.) of the internal combustion engine in a direction to suppress the knocking.
2. Description of the Related Art
In general, in internal combustion engines, a mixture of air and fuel introduced into a combustion chamber of each cylinder is compressed by an ascending movement of a piston received therein, and in an explosion stroke, the compressed mixture is fired and combusted by a spark on a spark plug which is generated by impressing a high voltage to the spark plug in the combustion chamber, whereby explosion energy at this time is taken out as a depression force of the piston and is converted into a rotational output.
When combustion is performed in the combustion chamber of each cylinder in the explosion stroke, molecules of the mixture in the combustion chamber are electrically dissociated (ionized), so when a high voltage is impressed, immediately after the explosion stroke, to electrodes for detection of an ionic current which are installed in the combustion chamber, ions with electric charge thus generated flow as an ionic current. In addition, it is known that the ionic current changes sensitively in accordance with the combustion state of the combustion chamber, and hence, the combustion state (occurrence of a misfire or a knock) in the cylinder can be determined by detecting the state of the ionic current.
Accordingly, there has conventionally been proposed an apparatus that can detect the occurrence of a knock in an internal combustion engine by detecting the state of an ionic current (see, for instance, a first patent document: Japanese patent application laid-open No. H10-9108).
In such a known knock control apparatus for an internal combustion engine described in the above-mentioned first patent document, a frequency band corresponding to the knock is extracted from the ionic current as a knock signal by means of a band-pass filter, and the knock signal is compared with a predetermined level to provide knock pulses, based on the number of which it is determined whether knocking has occurred.
In case where in the ionic current there occurs noise which has a frequency approximate to that of a knock and is able to pass through the bandpass filter, pulses corresponding to the noise are generated and detected, in view of which the number of the pulses thus detected is averaged to obtain an average number of pulses, which is subtracted from the number of knock pulses to provide a number of pulses corresponding to the knock, by which a control amount of retard angle is increased.
In addition, there has also been proposed an apparatus that is constructed so as to avoid noise generated upon seating of engine operating valves when vibration generated upon occurrence of a knock is detected by a knock sensor (see, for example, a second patent document: Japanese patent application laid-open No. H6-147079).
In the knock control apparatus for an internal combustion engine described in the above-mentioned second patent document, a first detection knock signal, when being within a set level range, is assumed to be valve seating noise, and by focusing attention on a cylinder for which the knock detection signal was obtained, a reoccurrence detection period timer is started to be driven from that point in time for counting a predetermined period Ts that is preset so as to detect the recurrence of noise. When the following knock detection signal, being within the set level range, is detected from the same cylinder before the reoccurrence detection period timer completes the counting of the period Ts, the knock detection signal is determined as valve seating noise and retard angle processing corresponding to the knock is cancelled.
It is known that noise of the same vibration component as a knock frequency might sometimes be superposed on the ionic current, depending upon the operating state of the internal combustion engine, in spite of the non-occurrence of a knock.
In addition, it is also known that in some engines, the pressure in a cylinder is sometimes caused to pulsate without regard to the occurrence of a knock, so a vibration component might be superposed on the waveform of an ionic current in accordance with the generation of such pressure pulsation. Further, the pulsating noise due to the cylinder internal pressure is misjudged as the occurrence of a knock, so ignition timing is correctively set to a retard angle side, and the supply of fuel is also correctively set to a rich side, as a result of which it is experimentally known that the frequency of occurrence of pulsating noise and the amplitude strength of vibration thereof both tend to increase.
With the conventional knock control apparatuses for an internal combustion engine, in case where a knock vibration component is extracted by using a band-pass filter as described in the above-mentioned first patent document, there is a problem that noise with the same frequency component as a knock frequency of a knock is not able to be distinguished from the knock.
Moreover, there is another problem as stated below. That is, it is very difficult to extract only a knock signal on which a noise component with a vibration amplitude strength and a vibration duration that are comparable with the detection level of a large knock upon occurrence thereof is superposed, which becomes an obstacle to the development of a knock detection apparatus particularly using an ionic current detection system.
Also, in the case of avoiding pulsating noise of cylinder internal pressure as in the second patent document, ignition timing need be correctively set to a retard angle side, and the supply of fuel need also be correctively set to a rich side due to the pulsating noise, so the frequency of occurrence of pulsating noise and the amplitude strength of vibration thereof both tend to increase. As a result, there exists noise, such as seating noise of valves of the valve operating system of the engine, which does not fall into the set level range for masking, and hence there is a problem that it is after all impossible to avoid the incorrect detection of pulsating noise.
Further, when a subsequent knock detection signal, being within the set level range, is detected from the same cylinder, it is mistakenly determined that the knock detection signal is valve seating noise, and retard angle correction processing for ignition timing is canceled in spite of a knock occurrence state, so there is a problem that it is impossible to suppress knocks occurring in succession.